1. Field of the Invention
The present invention relates generally to an electric power supply to convert an ac voltage to a stabilized dc voltage. More specifically, the invention relates to a switching power supply capable of improving a power factor thereof by substantially synchronizing a wave form and a phase of both an input voltage and an input current of a chopper circuit.
2. Description of the Related Art
One example of a switching power supply of power factor improvement type is proposed in a reference, Japanese Patent Laid-open Publication No. 4-168,975, which was filed by the present applicant. As shown in FIG. 4, the reference discloses a power supply having a configuration in which the output of a rectifying circuit 10 for rectifying in full wave, is fed to a boost chopper circuit, thereby a stabilized dc output voltage is obtained.
The chopper circuit in the reference comprises a switching device Q1 driven with sufficiently higher frequency than that of the input ac voltage, an inductor L1 connected in series with the switching device Q1 between output terminals of the rectifying circuit 10, and a diode D1 and a capacitor C1 connected in series between the input and output terminal of the switching device Q1 in order to let an output current flow through the inductor L1 during a turn off time of the switching device Q1. The capacitor C1 has substantially large capacity so that a smoothed and stabilized dc output voltage is supplied therefrom.
An error voltage between an output voltage V2 of the chopper circuit and a reference voltage Vs is detected by an error amplifier 11. The output signal of the amplifier 11, an error signal .DELTA.V, is input to a multiplier 12. An input voltage V1 (i.e., a full-wave rectified input ac voltage) of the chopper circuit is also input to the multiplier 12. The multiplier 12 multiplies V1 of the chopper circuit by .DELTA.V to generate and output a threshold level signal S0 with a full-wave rectified wave form, of which phase is same as that of the input voltage V1 of the chopper circuit, and an amplitude corresponding to the error signal .DELTA.V of the output voltage V2.
The instantaneous value of the current through the switching device Q1 in the chopper circuit is detected by a current sensing resistor R1. A current detection signal S1 measured as a voltage drop of R1 is compared with the threshold level signal S0 by a comparator 13. During a turn on time of Q1, the current through Q1 via the inductor L1 is kept increasing gradually. The current detection signal S1 also increases along with the Q1 current increase and the output signal of the comparator 13 turns HIGH when S1 reaches S0 to trigger a monostable multivibrator 2.
The monostable multivibrator 2 so functions that the output signal thereof is maintained LOW during a constant short time period dt from the trigger point by the output of the comparator 13. After the time period dt, the output signal is set HIGH. The switching device Q1 is so controlled by the monostable multivibrator 2 via a driver 15 that the turn on time of Q1 corresponds to HIGH level of the output of the monostable multivibrator 2, while the turn off time of Q1 corresponds to LOW level of the multivibrator output.
As shown in FIG. 5, the current flowing to the output side from the inductor L1 via the diode D1 gradually decreases after the switching device Q1 turns off.
The OFF time period dt of the switching device Q1 provided by the multivibrator 2 is sufficiently set small so that Q1 is turned on when the current through the inductor L1 is reduced below the threshold level S0 by a certain amount. For this reason, the instantaneous value of the L1 current does not become equal to zero level at its lowest points.
In other words, the switching device Q1 turns on when the current through the inductor L1 decreases by a certain amount, then the current through the inductor L1 and the switching device Q1 gradually increases. At the moment that the current reaches the threshold level S0, Q1 turns off and the L1 current starts to decrease. Repetition of this sequence enables a high frequency ON-OFF switching of the switching device Q1, which is sufficiently higher than the AC input frequency, so that an envelope of the current through the inductor L1 conforms to the threshold level signal S0 having a full-wave rectified wave form.
According to the above construction, since the L1 current varies to follow the threshold level signal S0 without becoming equal to zero level, the peaks of the current of the inductor L1 are sufficiently suppressed.
Furthermore, the switching frequency of the switching device Q1 varies along with the fluctuation of the ON time period of Q1, thus noise spectrum from the chopper circuit is diffused so that noise suppression can be easily accomplished.
In accordance with the above described conventional switching power supply, however, the switching frequency may vary excessively in practical aspect even though advantage of easy noise suppression is still assured. This results in that the duty ratio of the Q1 switching cannot be controlled in a wide range which is sufficient to allow the input voltage and the input current to vary in a same wave form and a same phase. Consequently, the problem may arise that the power factor of the power supply cannot be improved successfully.
Particularly, such a power supply producing a constant dc output of e.g., 380 V for an input ac voltage of, for example, 85 to 264 V may require a broader duty ratio for the switching device.